Novel tryptophan catabolic enzyme IDO2 is the preferred biochemical target of the antitumor indoleamine 2,3-dioxygenase inhibitory compound D-1-methyl-tryptophan.

Small-molecule inhibitors of indoleamine 2,3-dioxygenase (IDO) are currently being translated to clinic for evaluation as cancer therapeutics. One issue related to trials of the clinical lead inhibitor, D-1-methyl-tryptophan (D-1MT), concerns the extent of its biochemical specificity for IDO. Here, we report the discovery of a novel IDO-related tryptophan catabolic enzyme termed IDO2 that is preferentially inhibited by D-1MT. IDO2 is not as widely expressed as IDO but like its relative is also expressed in antigen-presenting dendritic cells where tryptophan catabolism drives immune tolerance. We identified two common genetic polymorphisms in the human gene encoding IDO2 that ablate its enzymatic activity. Like IDO, IDO2 catabolizes tryptophan, triggers phosphorylation of the translation initiation factor eIF2alpha, and (reported here for the first time) mobilizes translation of LIP, an inhibitory isoform of the immune regulatory transcription factor NF-IL6. Tryptophan restoration switches off this signaling pathway when activated by IDO, but not IDO2, arguing that IDO2 has a distinct signaling role. Our findings have implications for understanding the evolution of tumoral immune tolerance and for interpreting preclinical and clinical responses to D-1MT or other IDO inhibitors being developed to treat cancer, chronic infection, and other diseases.

Cyclin B1 is a critical target of RhoB in the cell suicide program triggered by farnesyl transferase inhibition.

Farnesyl transferase inhibitors (FTIs) have displayed limited efficacy in clinical trials, possibly because of their relatively limited cytotoxic effects against most human cancer cells. Therefore, efforts to leverage the clinical utility of FTIs may benefit from learning how these agents elicit p53-independent apoptosis in mouse models of cancer. Knockout mouse studies have established that gain of the geranylgeranylated isoform of the small GTPase RhoB is essential for FTI to trigger apoptosis. Here we demonstrate that Cyclin B1 is a crucial target for suppression by RhoB in this death program. Steady-state levels of Cyclin B1 and its associated kinase Cdk1 were suppressed in a RhoB-dependent manner in cells fated to undergo FTI-induced apoptosis. These events were not derivative of cell cycle arrest, because they did not occur in cells fated to undergo FTI-induced growth inhibition. Mechanistic investigations indicated that RhoB mediated transcriptional suppression but also accumulation of Cyclin B1 in the cytosol at early times after FTI treatment, at a time before the subsequent reduction in steady-state protein levels. Enforcing Cyclin B1 expression attenuated apoptosis but not growth inhibition triggered by FTI. Moreover, enforcing Cyclin B1 abolished FTI antitumor activity in graft assays. These findings suggest that Cyclin B1 suppression is a critical step in the mechanism by which FTI triggers apoptosis and robust antitumor efficacy. Our findings suggest that Cyclin B1 suppression may predict favorable clinical responses to FTI, based on cytotoxic susceptibility, and they suggest a rational strategy to address FTI nonresponders by coinhibition of Cdk1 activity.

Genetic response to DNA damage: proapoptotic targets of RhoB include modules for p53 response and susceptibility to Alzheimer's disease.

Knockout mouse studies indicate that the small GTPase RhoB is critical for apoptosis triggered by genotoxic stress in transformed mouse cells. However, the mechanisms used by RhoB to sensitize cells to cell death are obscure. To gain insight into this question, we compared the genetic response of cells with different rhoB genotypes to the DNA damaging anticancer drug doxorubicin (DOX). The microarray hybridization strategy focused on events occurring by 6 hr of DOX treatment, preceding the execution phase of RhoB-dependent apoptosis by 12-16 hr. Genes controlling cytoskeletal organization, adhesion, transcription, trafficking, apoptosis, and protein turnover were represented prominently. Gene clustering revealed a module of p53 target genes, suggesting that RhoB may modify the p53 response, and a module for susceptibility to Alzheimer's disease, suggesting a link between RhoB and age-associated dementia. The findings of this study suggest mechanisms by which RhoB may act to elevate the sensitivity of cells to apoptosis following genotoxic stress.

Genetic response to farnesyltransferase inhibitors: proapoptotic targets of RhoB.

Knockout mouse studies have established that the transformation-selective death program triggered by farnesyltransferase inhibitor (FTI) requires a gain-of-function in the stress-regulated small GTPase RhoB. To gain insight into this death program, we compared the genetic response of cells with different RhoB genotypes to FTI treatment. The microarray hybridization strategy we employed focused specifically on events preceding the execution of RhoB-dependent apoptosis, which is crucial for effective antineoplastic responses in mouse, rather than on other aspects of the FTI response mediated by RhoB gain-of-function (e.g., growth inhibition). Genes that control cell adhesion and cell shape were represented prominently among upregulated targets, as were genes that control signal transduction, vesicle dynamics, transcription, and immunity. Genes that control cell cycle checkpoints and progression through S phase and mitosis were among the major downregulated targets. In support of the concept of RhoB as a negative regulator of Ras signaling pathways, the most strongly downregulated gene scored was farnesyl pyrophosphate synthetase, the enzyme that produces the substrate used by FT to farnesylate Ras proteins. Gene clustering revealed modules for MAPK signaling, cell cycle progression, and immune response as proapoptotic targets of RhoB. This report identifies genes that pertain to the transformation-selective apoptotic program triggered by FTI. Further study of this program may yield insights into the dramatic differences in efficacy and apoptotic prowess of most FTIs in human cancers, versus transgenic mouse models.

mDia function is critical for the cell suicide program triggered by farnesyl transferase inhibition.

Farnesyl transferase inhibitors (FTIs) exhibit limited cytotoxic effects against human cancer cells, perhaps explaining the limited efficacy of FTIs in clinical trials. Learning how these well-tolerated drugs trigger p53-independent apoptosis in mouse models of cancer might therefore benefit efforts to leverage their utility in clinic. Recent clinical findings indicate that the oncogenic Rho guanine nucleotide exchange factor AKAP13/Lbc is associated with clinical responsiveness, in support of an earlier genetic proof in mice that gain of the geranylgeranylated isoform of RhoB which blocks oncogenic Rho signaling is essential for FTI-induced apoptosis. Here we offer evidence that the RhoB effector mDia is a critical downstream player in this death program. Dominant inhibition of mDia ablated FTI-induced apoptosis but not actin reorganization or growth inhibition, the latter of which has been linked previously to interactions with a RhoB effector kinase pathway that downregulates c-Myc. In nude mice, dominant inhibition of mDia promoted tumor formation and ablated FTI antitumor efficacy. Our findings suggest that the RhoB-mDia pathway is critical for the cell death mechanism engaged by FTI. Further, they suggest that mDia may be important for Rho-dependent survival of oncogenically transformed cells, perhaps driven by AKAP13/Lbc.

Salinity stress-tolerant and -sensitive rice (Oryza sativa L.) regulate AKT1-type potassium channel transcripts differently.

In the indica rice (Oryza sativa L.) a cDNA was characterized that encoded OsAKT1 homologous to inward-rectifying potassium channels of the AKT/KAT subfamily. Transcript analysis located OsAKT1 predominantly in roots with low abundance in leaves. Cell-specificity of OsAKT expression was analyzed by in situ hybridizations. In roots, strongest signals were localized to the epidermis and the endodermis, whereas lower transcript levels were detected in cells of the vasculature and the cortex. In leaves, expression was detected in xylem parenchyma, phloem, and mesophyll cells. Transcriptional regulation and cell specificity of OsAKT1 during salt stress was compared in rice lines showing different salinity tolerance. In the salt-tolerant, sodium-excluding varieties Pokkali and BK, OsAKT1 transcripts disappeared from the exodermis in plants treated with 150 mM NaCl for 48 h but OsAKT1 transcription was not repressed in these cells in the salt-sensitive, sodium-accumulating variety IR29. Significantly, all lines were able to maintain potassium levels under sodium stress conditions, while sodium concentrations in the leaves of IR29 increased 5-10-fold relative to the sodium concentration in BK or Pokkali. The divergent, line-dependent and salt-dependent, regulation of this channel does not significantly affect potassium homeostasis under salinity stress. Rather, repression in Pokkali/BK and lack of repression in IR29 correlate with the overall tolerance character of these lines.

Characterization of a HKT-type transporter in rice as a general alkali cation transporter.

We report the characterization of rice OsHKT1 (Oryza sativa ssp. indica) homologous to the wheat K+/Na+-symporter HKT1. Expression of OsHKT1 in the yeast strain CY162 defective in K+-uptake restored growth at mM and micro M concentrations of K+ and mediated hypersensitivity to Na+. When expressed in Xenopus oocytes, rice OsHKT1 showed uptake characteristics of a Na+-transporter but mediated transport of other alkali cations as well. OsHKT1 expression was analysed in salt-tolerant rice Pokkali and salt-sensitive IR29 in response to external cation concentrations. OsHKT1 is expressed in roots and leaves. Exposure to Na+, Rb+, Li+, and Cs+ reduced OsHKT1 transcript amounts in both varieties and, in some cases, incompletely spliced transcripts were observed. By in situ hybridizations the expression of OsHKT1 was localized to the root epidermis and the vascular tissue inside the endodermis. In leaves, OsHKT1 showed strongest signals in cells surrounding the vasculature. The repression of OsHKT1 in the two rice varieties during salt stress was different in various cell types with main differences in the root vascular tissue. The data suggest control over HKT expression as a factor that may distinguish salt stress-sensitive and stress-tolerant lines. Differences in transcript expression in space and time in different lines of the same species appear to be a component of ion homeostasis correlated with salt sensitivity and tolerance.